The performance of ultra capacitor and lithium battery electrolytes at low temperature is a continuing problem since the conductivity of the electrolyte will go to zero if it freezes before a desired low temperature performance is achieved. Various blends of organic carbonates have been used along with the addition of ethers and low molecular weight esters to achieve low temperature (−60° C.) freezing points of the mixed solvents containing lithium hexafluorophosphate for low temperature lithium-ion battery performance. The use of mixtures of ethylmethyl carbonate with dimethyl carbonate and small quantities of ethylene carbonate have allowed performance down to −20° C. and even −30° C. in some cases for lithium battery electrolytes. The use of tetrahydrofuran (THF) and methyl formate and methyl acetate and dimethyl ethylene glycol (glyme) or dimethoxy ethane (DME) has allowed some battery electrolytes to achieve −40° C. or even −50° C. performance. The problem is that the performance of these electrolytes at high temperatures such as >70° C. causes high vapor pressures in the batteries with these volatile low boiling solvents. In the case of ultra capacitors based on organic electrolytes the situation is similar except most current ultra capacitor electrolytes are based on the use of acetonitrile (low boiling, with by 82° C.) containing tetraethylammonium tetrafluoroborate.
These ultra capacitor electrolytes have an upper operating voltage limitation of 2.7 V. The use of capacitor electrolyte solvents based on the organic carbonates and containing tetralkylammonium tetrafluoroborates has also been limited because of solubility limitations when using propylene carbonate, or by low temperature performance when using mixtures containing ethylene carbonate.
Propylene carbonate has been used in mixtures with other organic carbonates for capacitor electrolytes but this solvent also limits cell voltage to about 3 V and the solubility of the tetrafluoroborate salt decreases rapidly on cooling and also results in low conductivity of the electrolyte at temperatures below −20° C. The use of ethylene carbonate (mp 35° C.) with cyclic organic carbonate mixtures containing organic quaternary tetrafluoroborate salts for capacitor use gives higher operating cell voltage in ultra capacitors, but these electrolytes freeze before −20° C. is reached. Low temperature cycling performance (non-freezing) is desired (required for the use of ultra capacitor performance in vehicle performance down to −30° C.). In aircraft, the temperature desired is down to −40° C. At the same time these applications desire high temperature performance >70° C. with low vapor pressure. This means that volatile solvents which are used for low temperature performance cause problems at the high end of the desired performance range.
U.S. Pat. No. 6,535,373 to Smith, et al., which is herein incorporated by reference, relates to non-aqueous electrolyte solutions containing quaternary ammonium tetrafluoroborate salts that can be used in the present invention. The solvent used in the patent is a nitrile solvent.
U.S. Pat. No. 7,924,549 to Smith, et al., which is herein incorporated by reference, relates to carbon electrodes for capacitors with a high concentration of tetrafluoroborate salts in a non-aqueous aprotic solvent.
U.S. Pat. No. 6,980,415 to Higono, et al., which is herein incorporated by reference, discloses an electrolyte for capacitors comprising dimethyl carbonate and a spiro tetrafluoroborate salt. The solvent and the tetrafluoroborate salt can be used in the present invention.
U.S. Patent Publication No. 20070194266 to Chiba, et al. which is herein incorporated by reference, discloses an electrolyte solution for electric double layer capacitors comprising quaternary ammonium salts and ethylene and propylene carbonate as an electrolyte.
An electrolyte solution which contained an unsaturated carbonate solvent of less than 8% by weight and a saturated solvent of less than 5% by weight has been reported (U.S. Patent Publication No. 2007/0224514 to Kotato et al.). The reason for the low amounts of these two solvent types is that “if the volume exceeds the upper limit an excessive amount of negative electrode coating may be formed and prevent the migration of lithium ions.”